Apparatus for indurating ore particles



Nov. 29, 1966 R. J. LINNEY APPARATUS FOR INDURATING ORE PARTlCLES 6 Sheets-Sheet 1 Original Filed June lO, 1964 IMPN Wma

Nov. 29, 1966 R. J. LINNEY APPARATUS FOR INDURATING ORE FARTICLES 6 Sheets-Sheet 2 Original Filed June lO, 1964 Original Filed June lO, 1964 Nov. 29, 1966 R. J. LINNEY 3,288,449

APPARATUS FOR INDURATING ORE PARTILES 6 Sheets-Sheet 3 74@ l y f "74 \75 75 A91 \/l l 34 1 l I l INVENTOR. v .PosE/PrLf//v/VEK ffl/Lof, @amil Mv NOV. 29, 1966 R 1, INNEY 3,288,449

APPARATUS FOR INDURATING ORE PARTICLES INVENTOR.

oE/QTJ UvA/Ex BY Nov. 29, 1966 R. J. LINNEY APPARATUS FOR INDURATING ORE PARTICLES G Sheet S-Shee i,

Original Filed June l0, 1964 MWQRNTS m j WM) Nov. 29, 1966 R. J. UNNEY 3,288,449

APPARATUS FOR INBURATING ORE PARTICLES Original Filed June 10, 1964 6 Sheets-Sheet 6 me/wim United States Patent O 3,288,449 APPARATUS FOR INDURATENG ORE PARTICLES Robert J. Linney, Shaker Heights, thio, assigner to Reserve Mining Company, Silver Bay, Minn., a corporation of Minnesota Original application .lune 10, 1964, Ser. No. 374,119, now Patent No. 3,244,507, dated Apr. 5, 1966. Divided and this application June 11, 1965, Ser. No. 463,281 4 Claims. (Cl. 263-28) This application is a division of my copending application Serial No. 374,119, filed June 10, 1964, now U.S. Patent No. 3,244,507, which in turn is a continuation-inpart application of the following copending applications: Serial No. 154,308, filed November 22, 1961, now abandoned; Serial No. 849,811, filed October 30', 1959, now abandoned; and Serial No. 682,826, tiled September 9, 1957, now abandoned.

Thisjnvention pertains to methods and apparatus for agglomerating finely comminuted ores and ore concentrates into pelletzed form and for indurating the pellets by heat treatment for imparting thereto high hardness, strength, and resistance to fragmentation in handling or shipping.

As a convenient example, the present invention is exemplified with respect to pelletizing concentrates of taconite ores. However, it is not limited thereto but is applicable to the agglomeration of all types of finely comminuted ores and ore concentrates into pelletized form.

The invention finds particular -application in the agglomeration yand indurated pelletization of oxidic iron ores and concentrates and, more especially to -pulVerulent concentrates of the taconite ores, containing the magnetic oxide, Fe304, which undergoes during the indurating heat treatment of the invention, an exothermic oxidation to the non-magnetic or hematitic oxide, Fe203, with resultant recrystallization and grain growth between discrete particles within each pellet which firmly bonds them together into an integrated pelletzed structure of extremely high strength, hardness and correspondingly high resistance to fragmentation when subjected to the pressures and shock impacts incurred during loading, shipping, unloading orvotherwise conveying or handling.

The taconites, being low grade iron ores, cannot economically 'be reduced as such in the blast furnace and hence must be beneficiated by crushing, grinding and magnetic separation techniques until a finely comminuted 'concentrate tof for example 60-65% iron content is obtained. This powdery product is, however, unsuitable for treatment in blast furnaces, open hearth furnaces and the like, a-nd hence must be agglomerated into aggregates or lumps of suitable size and `of sufficient strength for such applications. Various yprocedures to this end have heretofore been proposed such as briquetting, sintering, pelletizing, et-c. Briquetting techniques are, however, expensive as t-o equipment and power requirements, and the briquetted product is at best quite frangible. Also certain ore `and concentrate fines cannot be effectively briquetted without the use of binders in character and amount objectionable in subsequent reduction. Sintering is objectionable .on various grounds, such as excessive fuel consumption in processing and excessive bulk of the product.

Pelletization as heretofore proposed or practiced has likewise yproved objectionable in various respects. Shaft kilns have been employed for this purpose, but owing to the excessive depth land volume of the pel-let bed, are difficult to control as to uniformity of feeding and temperature of heating of the pellets, with resultant formation of sintered pellet aggregates, obstructive of gas flow ice and productive of overburned aggregates. Also the green pellets entering the kilns tend to be crushed and slurried by the pressures encountered and moisture condensation from the kiln gases.

Travelling grate processes for pellet induration have likewise been proposed but none -prior to my invention has proved commercially successful insofar as I am aware, this by reason of defects inherent therein. It has heretofore been proposed to indurate a bed of green pellets fed through an ignition zone on a travelling grate conveyor, by direct iiame impingement upon the pellet bed and penetration thereof by flames from fuel-fired burners disposed above the bed. l have found this to be highly objectionable, for one reason because direct flame impingement on the bed causes the t-op layer of pellets to be fused together into a mass which is objectionable for this reason, and also for the further reason that this fused mass has poor porosity and thus retards the flow of gas through the bed and thereby likewise prevents proper induration of the pellets at the lower bed levels.

As applied particularly to induration of pellets agglomerated from taconite ore concentrates, direct iiame impingement on the pellet bed is further objectionable for reasons as follows. My investigations have established that the strongest and finest quality iron ore pellets derived from taconite concentrates, are produced when the temperature and oxidation conditions are such that the magnetite grains are oxidized to 'hematite and sufficient grain growth occurs after this oxidation and re-crystallization to form an almost complete network of interconnecting hematite grains, as illustrated in the accom-panying drawings as discus-sed below. The total strength of the pellet is related to the bridging of the hematite particles as the resul-t of the recrystallization and grain growth process. This ideal pellet structure is obtained by controlling the actual temperature of pellet induration at about 2300 to 2350" F. and below 2400 F.

In contrast to this, the temperature of oxidizing flames from combustion of normal commercial fuels, i.e., gas,

' oils or pulverized coals, is about 2700 -to 3000 F. Di-

rect impingement of such a flame on a bed `of pellets heats some of the pellets to so high a temperature that oxidation of the magnetite to hematite does not occur and the slag formation is so extensive that the grains of magnetite are embedded in a brittle slag matrix, also illustrated in the accompanying drawings `as discussed below. The sotreated pellets by reason of their resultant brittleness are easily fragmented, and thus quite inferior to pellets indurated free from flame impingement.

Now in accordance with an important aspect of my invention, I provide a travelling grate apparatus for progressively hardening a bed of moist green pellets of ore or ore concentrate fines, and in which, after drying they are subjected to a 'hardening heat treatment during no stage of which are the pellets subjected to direct liame impingement from fuel burners or other heating sources. In my apparatus the pellets are hardened by forced flow of hot gases through the bed at a temperature such as to heat the pellets to temperature within the range of about Z300-2400" F. and below that productive of fusion or sintering.

A further basic objection to travelling grate type pelletizing systems as heretofore proposed, resides in the thermally inefiicient manner in which they are arranged and operated with resultant excessively high fuel consumption. In such systems oxygen burned fuel, such as oil or gas, is burned throughout the length of zone -required for preheating the pellet bed to indurating temperature and f-or maintaining the bed temperature thereat until induration or hardening of the pellets is completed, Whereupon the hot pellet bed is immediately cooled to low temperature by a forced draft of air at atmospheric temperature. Although the combustion gases after passing through the bed in the indurating zone are at a temperature of about l000 F., they are nevertheless immediately discharged to atmosphere as waste gases. This is due to the inept arrangement of compartments wherein waste gases and gases carrying sensible heat are exhausted into a common chamber, necessitating their discharge to atmosphere. Such operation is therefore exceedingly wasteful of fuel, since -only a small fraction of the chemical heat values of the fuel is usefully employed in hardening the pellets.

In accordance with a further important aspect of my invention, I largely eliminate this defect by employing the heat from oxygen burned fuel only over a relatively short traverse of the pellet bed, just suiiicient to preheat a two to three linch band of the bed to indurating temperature to initiate the hardening action therein, following which the bed is fed through .a downdraft reaction zone wherein the induration band is caused to progress down through the bed by air preheated to about 800 F. as derived from a subsequent updraft reaction and cooling zone through which the pellet bed is fed and wherein induration is completed and the bed is cooled by a forced draft of ambient air which is thus preheated in passing through the hot pellet bed. In the induration of pellets composed of taconite ore concentrates the recuperative induration heating obtained in this way is of course supplemented Iby the exothermic heat resulting from oxidation of magnetite present in the ore particles to hematite as above explained.

I find that as a result of the heat conservation thus obtained in my process, that the fuel energy consumption is only about 450,000-500,000 B.t.u.s per gross ton of pellets processed as compared -to about 700,000-750,000 B.t.u.s per gross ton of pellets processed in accordance with the prior art proposals above discussed. The saving on fuel consumption is accordingly about 33 percent with my process.

I have also found that as a result of lfirst passing the pellets through a downdraft reaction zone and thence through an updraft zone there is, in effect, a double induration of the pellets and substantial improvements in pellet quality as compared to pellets processed in accordance with prior art proposals above discussed. For example, in tests of the product from my process as measured Iagainst the product from prior art proposals, I found that my product had: (a) a 3.6% higher porosity which resulted in a 6.8% faster deoxidation rate and a subsequent increase in production in a blast furnace; (b) an increase of 14.2% in crushing strength and 10.5% less -28 mesh ines after a standard tumbler test which indicates that my product is delivered to the blast furnace with a greater percentage of whole pellets and a lesser percentage of fine material, thereby allowing greater amounts of air, with less pressure drop, to How through the charge in a blast furnace and hence increasing production; (c) a decrease of 48% in residual ferrous iron due to a more complete oxidation of the magnetite to hematite as a result of the secondary induration in the updraft reaction and cooling zone wherein the primary induration band which has progressed down to the grate in the downdraft reaction zone is caused to reverse direction by the updraft of ambient air in the updraft reaction and cooling zone and thus migrate up through lthe pellet bed before the sensible heat contained therein is given up to heat the air used for recuperation. This secondary induration also provides for additional grain growth, thereby increasing pellet strength.

According to my invention only about one-half of the preheated Iair from the updraft lreaction and cooling zone is required for primary induration of the pellets in the downdraft reaction zone, the balance being available and utilized for supplying preheated air to the combustion heating zone and also to a downdraft drying zone through which the pellet bed is fed for drying the moisture-laden, green pellets prior to heating to indurating temperature.

Having thus generally described the basic features of my invention, reference will now be had for a more detailed description of these and other features to the accompanying drawings, wherein:

FIG. la is a simplified, diagrammatic showing in longitudinal elevation of a preferred apparatus for practicing my invention and which also illustrates my novel process as to operational sequences.

FIG. 1b is a temperature profile of gas temperatures at successive stages of the FIG. 1a sequence of operations.

FIG. 2 is a more or less diagrammatic showing in longitudinal sidel elevation of a preferred form of apparatus for practicing the invention commercially. FIG. 3 is a transverse sectional elevation as taken substantially at 3 3 of FIG. 2. FIG. 4 is a transverse sectional elevation as taken substantially at 4-4 of FIG. 2.

FIG. 5 is a diagrammatic showing in longitudinal side elevation ofthe indurating band commencing in the downdraft combustion heating zone, thence down through the pellet bed in the downdraft reaction zone and the reversal of direction up through the pellet bed in the updraft reaction and cooling zone, thus illustrating my novel double induration as to operational sequence.

FIG. 6 is a side elevation of a counterweighted air seal for isolating and maintaining positive and negative pressures between updraft and downdraft zones as practiced in my invention.

FIGS. 7 and 8 are p-hotomicrographic showings of the pellet structure after induration of pellets of taconite ore concentrates as indurated in accordance with the invention versus induration in accordance with the prior art proposals above discussed.

Referring to FIG. la, there is shown a gas-permeable, travelling grate, endless conveyor 10, fed about sprocket 11, which is driven for moving the grate in the direction of the arrow X. The grate is made of articulated links 13, provided with oppositely disposed sidewalls, which during horizontal travel of the Igrate, form continuous sidewalls, as at 14, for retention of the pellet bed, indicated at I. Moist green pellets are continuously deposited in a uniform layer onto the grate at the feed end, as at 1'5, from a vibrating feed screen 16, and indurated pellets being discharged from the delivery end of the grate, as at 17. The grate during its upper path of horizontal travel, is completely hooded and compartmentalized, as at 20-27, inc. Certain of the compartments are interconnected by conduits, as at 28, 29, provided with blowers, as at 30, 31, 32; while other compartments are provided with inlet or outlet conduits, as at 33, 34, 35, also provided with blowers, as at 36, 37, 38, for purposes explained below. A combustion heating chamber or furnace 39, is provided with fuel-fed sidewall burners 40, as discussed below.

In the operation of the pellet indurating process of my invention as illustrated in FIG. 1a, the bed P of the moist, green pellets as laid down `at 15 at the feed end of the travelling grate 10, is fed thence successively through the various treating zones indicated by the legends at the top of the drawing and comprising, updraft drying, downdraft drying, combustion heating, downdraft reaction and updraft reaction and cooling zones.

The only source of fuel-fired heat supplied to the system is by way of the fuel-fired burners 40 mounted in the sidewalls of the hooded chamber 39 comprising a combustion chamber or combustion zone. The hot gases from these burners are caused to flow in downdraft through the pellet bed and grate by the suction blower 37 in conduit 34 which connects to chamber 24 beneath the furnace chamber 39. Since these gases are deficient in oxygen due to combustion of fuel in the burners 40, they are discharged as waste gases to atmosphere via conduit 34 depleted of their sensible heat.

These hot gases in passing through the pellet bed from chamber 39 to 24, heat up the top tw-o or three inches of the bed of .pellets to indurating temperature in the manner illustrated at A, FIG. 5. As the bed of pellets is conveyed thence across the downdraft reaction zone 22, 23, this band of induration which started on top of the bed progresses down through the bed as shown at B, FIG. 5, until the two to four inch band reaches the grate 4l, as at C, just before the pellets are conveyed into the updraft reaction and cooling zone 21, 20.

The recuperation is initiated in chamber 20 wherein the hot pellets are subjected to an updraft of cool, ambient air forced into the lower cooling chamber 20 via conduit 33 and forced draft blower 36, FIG. 1a. This updraft of cool ai-r impinges iirst on the conveyor grate bars, as at 41, FIG. 5, thus to cool the same to prevent warpage and distortion, and the heat from which is caused to ow up through the pellet bed along with cool air, as at D, FIG. 5, thereby reversing the direction of the indurating band causing it to migrate up through the bed and at the same time cooling the bed beneath the indurating band and also heating the air flowing into upper chamber 21. As shown by the gas temperature diagram of FIG. lb, the thus heated .air enters chamber 21 .at a temperature of about y800 F., while meantime completing induration and cooling the `grate bars and pellet bed down to temperature of about 200 F. for discharge.

The upper reaction and cooling chamber 21 opens into the upper downdraft reaction chamber 23, las .indicated by the arrow, and a portion, preferably about one-half of the 800 F. ai-r flowing into chamber 21, is caused to ilow into the upper downdraft reaction zone chamber 23 and thence in downdraft through the pellet bed in the downdraft reaction zone by virtue of the suction blower 31 in conduit 29, FIG. 1a, connected to the lower chamber 22 of the downdraft reaction zone.

The thus preheated air from chamber 21 in passing in downdraft through the pellet bed completes primary in- -duration of the pellets initiated in the combustion heating zone. As shown by FIG. 1b, the downdraft air after passing through the pellet bed -in the downdraft reaction zone and after tempering in the manifold 22, has a temperature of about 800 F. It is fed via conduit 29 and blower 31, thence through a second :blower 32 int-o the lower d-rying zone chamber V26, .and thence in updraft through the travelling grate and pellet bed into the upper drying Zone chamber 27, from whence moisture-laden, it is discharged to atmosphere via conduit 35 containing the suction blower 38, at temperature of about 200 F., as shown by reference to FIG. 1b.

The hot dry air entering the lower drying zone charnber 26 at about 800 F., in passing in updraft through the lpellet bed, is thus most effective in drying the pellets in the lower portion of the bed thereby to green harden and .strengthen the same, `and it also provides a supporting column of uprising air to prevent crushing by the upper laye-r of pellets. Also in this way such moisture condensation as occurs on the cold pellets will do .so only on the upper layer of pellets having minimumload thereon, thereby to prevent injury by crushing.

My investigations have shown that if the hot ai-r is rst passed in downdraft through the cold, entering bed of pellets, that although the pellets in the upper layer of the bed Will be dried and green hardened, those in the lower layer will slurry and crush due to moisture condensation thereon from the now moisture-laden, heated air, which in passing through the upper pellet layer has picked up considerable moisture, and that the wet slurried pellets in the lower part of the bed will crush due to being subjected to the pressure of the downdraft column of air and the weight of the pellet bed.

Reverting to the discharge end 17 of the Igrate, as above stated, only about one-half of the heated air flowing into the upper updraft reaction and cooling zone chamber 21, is caused to iiow into the upper downdraft reaction zone chamber 23. The remainder is drawn off from chamber 21 over conduit 28 through the suction blower 30, and conveyed thence to the downdraft drying and combustion heating zone chambers 25 and 39. The portion entering the downdraft drying zone chamber 25, passes in downdraft through the pellet bed to further dry and green harden the pellets by removing residual moisture therefrom with particular reference tto the residual moisture contained in the upper pellet layer. The portion entering the combustion heating zone chamber 39, serves to provide preheated air to the burners 40 and also to the combustion heating zone chamber 39, and thus reduces to that extent the amount of thermal energy to be supplied by fuel consumed by the burners for heating the upper part of the pellet bed up to the indurating temperature of about 2300- 2400" F.

As above stated and as described more in detail below with reference to FIG. 3, the burners 40 are so mounted, flame-adjusted and positioned above the pellet bed, that there occurs no direct impingement of frame from the burners on the pellet bed. Only hot gases heated by the burners contact the bed being drawn therethrough as explained by the suction blower 37 in conduit 34 connected to the lower combustion Zone chamber 24. The temperature of these gases is so adjusted as to heat the upper layer of the pellet bed to indurating temperature olf about 23002400 F. and thus to initiate the indurating reaction as the bed passes into the downdraft reaction zone, wherein the primary induration reaction is completed in the manner above explained. That is to say, as the pellet bed traverses the downdraft reaction zone, a wave of heat at indurating temperature is caused to pass downwardly through the bed due to the forced flow downdraft of preheated air from chamber 21 passing through the bed while the exothermic reaction of the magnetite to hematite oxidation occurring therein aids in holding the pelletizing temperature at optimum.

As shown by FIG. lb the gases from the lower chamber 24 of the combustion heating zone exit to atmosphere at only about 400 F. This is in marked contrast to the prior `art systems above discussed wherein these gases exit at about 1000 F. This marked difference in temperature of exit waste gases in my process versus the prior art, results from the heat conservation obtained by recuperation in my process. This recuperation takes dry air at about 800 F. containing an oxygen level of 100% of ambient from chamber 21 as preheated by updraft reaction and cooling of the hot indurated pellets and employs the heat values thereof lfor primary induration of pellets in the downdraft reaction zone and thence for updraft drying in the updraft drying Zone, the air from which is exhausted to atmosphere at the low temperature of `about 200 F. In addition, blower 30 transports part of the 800 F. air from the updraft reaction and cooling chamber 21 to the combustion heating chamber 39, to the burners 4t), and to the downdraft drying zone 25, these gases passing thence through the pellet bed to atmosphere at 400 F. The prior art is devoid of any such .teaching or suggestion insofar as I am aware.

Referring now to the commercial embodiment of my invention as shown in FIGS. 2-6, inc., equivalent elements have been similarly designated as in FIG. la from which the identity of equivalent components will be apparent. However, there are some structural modifications in the FIGS, 2 6, inc., embodiment which will 'become apparent by comparison with the simplified showing of FIG. la. Thus, referring to FIGS. 2-6, inc., 4a series of funnel-like wind boxes, as at 50, 51, are disposed seriatum substantially the length of the travelling grate 10, and are positioned with the flared openings just beneath the grate, as at 52, 53. A series of manifolds 20, 22, 24, 26, corresponding in .function to the like-numbered lower chambers of FIG. laJ are disposed seriatum below the wind boxes as shown, and from each wind box a conduit extends downwardly to the manifold positioned beneath the same. Thus, conduits 54, 55, extend from wind boxes 50, 51,

down to manifold 26, positioned beneath them, and so on for the remaining wind boxes. The manifolds, accordingly function to segregate the wind boxes into groups corresponding to the lower like-numbered lower drying, combustion heating, downdraft reaction Iand updraft reaction and cooling compartments of FIG. la.

Above grate are disposed compartmentalized hoods 27, 25, 39, 23 and 21, corresponding to the like numbered upper hood compartments or chambers of FIG. la, for providing the upper drying, combustion heating, downdraift reaction and updraft reaction and cooling compartments. Likewise, in FIGS. 2-6, inc., the motor-driven blowers and conduits containing them are arranged and connected the same as the like-numbered components of FIG. la and for performing the same functions, respectively. Referring to FIGS. 3 and 4, oppositely disposed side panels 56 and 57, extend down from the upper hoods to hoppers 78, in order to enclose the grate 10 substantially throughout its length, thereby making the entire enclosure a pressure balancing plenum chamber similar to that described in U.S. Patent 3,088,723. This tunnellike housing encloses the grate from the upper hoods 21, 23, 39, 25 and 27 to below the return strand of the grate by walls 56 and 57 as shown in FIGS. 3 and 4. The cntire tunnel-like structure is under a slight negative pressure and acts as a vacuum cleaner to contain essentially .all of the air-borne dust in the system. The tunnel also acts as a pressure equalizing plenum chamber to contain the small surges of positive or negative pressure in the air ow system of the machine rather than allowing them to dissipate to atmosphere.

In addition to the above described dust collection system, dust collecting means are provided both at the feed end and discharge end of grate 10. Referring to FIG. 2, at the feed end, wind box 95 is connected to updraft drying chamber I27 by conduit 96. Wind box 95 is under slight negative pressure, being exhausted by blower 38, and picks up air-borne dust from the slight blow-by through air seal 94, FIGS. 2 and 6.

Referring to FIG. 6, the air seal mechanism shown generally at 94, comprises an air sealing plate member 100, pivotally mounted as at 101, to a supporting structure 102 of the wind box assembly, the air sealing plate member having integral therewith a counterweight 103, of suicient magnitude to maintain the sealing plate and counterweight assembly in the position shown by the dotted lines 10011, 103a, whereby the outer edge of the seal'ing plate rides along the underside of the pallets or articulated links 13 of the travelling grate conveyor 10, thus to minimize blowby of air between the sealing plate and the grate.

Reverting to FIG. 2, the discharge end of grate 10 is enclosed by dust collecting hood 97, whereby dust is exhausted to dust collectors, not shown, via conduit 98 and is eventually returned to the system. The blow-by in the terminal wind box is controlled to a minimum by air seal 91 similar to that shown in FIG. 6. Two additional and similar air seals 92 and 93, FIG. 2, effectively prevent the short-circ-uiting of air between the pressurized wind lbox on one side of the seal and the suction wind box on the other side of the seal.

As further shown in FIG. 2, damper controls are provided in the various conduits, as at 60, 61, 62, for controlrling the flow of air therethrough. There are also tempering air dampers, as at 67, 68 and 69, to permit ambient air to be drawn into the respective ducts or `chambers to control the temperature of the gases passing through blowers for their protection against excessive temperatures. Conduit 28 `is branched as at 63, 64, for directing desired fractions 4of the hot updraft air from chamber 21 into the combustion heating and downdraft drying chambers 39, 25, on the one hand, and as preheated air supply to the f-uel burners 40, on the other. The lower manifold 20 in the updraft reaction and cooling zone is provided with re- .movable sections, as at 65, 66, for increasing or decreasing the effective length of this manifold, and thereby correspondingly varying the effective length of the updraft reaction and cooling zone.

Referring to FIG. 3 the burners 40, are fitted as above stated into ports, as at 70, extending through sidewalls, as at 71, 72, of the combustion furnace chamber 39. By reason of this mounting the burners direct their flames horizontally into the com'bustion heating chamber 39 at a level Well above the pellet bed P of the travelling grate 10, in the manner illustrated at 73. Thus, there occurs no direct impingement of the flames 73 from the burners 40 onto the pellet bed P. The flames are additionally adjusted to assure this by controlling the rates of air and fuel supply to the burners over the air lines, as at 74, and fuel lines, as at 75. The air supplied to the air lines 74 is preheated air supplied over conduits 28 and 64, FIG. 2. The air fed into the combustion heating chamber through the inlet conduits 76 is the preheated air supplied thereto over conduits 28 and 63, FIG. 2.

The preferred pellet .size is about 1]/32 to 3A inch. The screen 16 which feeds the green pellet agglomerates into the feed end of the travelling grate is of a mesh to retain pellets of Mt inch but to reject smaller sizes. The grate bars of the travelling grate are likewise spaced to retain pellets of about a inch and over but to pass smaller sizes. Referring to FIGS. 3 and 4, pellets fragmented during feed over the travelling grate or those that fall through openings caused by broken grate bars, fall into hoppers, as at 77, 78, at the base of the wind box conduits, and are removed on travelling belts, as at 79, 80.

As was pointed out above, taconite ore pellets as indurated at about 2300-2400 F. in the absence of direct llame impingement on the pellet bed produce the strongest and finest quality iron pellets when the temperature and oxidation conditions `are such that the magnetite grains are oxidized to hematite and suicient grain growth occurs after this oxidation and recrystallization to form an almost complete network of interconnecting hematite grains. The microstructure of pellets as thus produced in accordance with the present invention is shown in FIG. 7 of the drawings. As there shown the microstructure consists of a well-bridged network of hematite grains comprising the white areas. It will be noted that the slag content comprising the gray areas is generally confined to coalesced patches within the individual grains or as longer isolated particles. The voids are shown in black.

It was further pointed out in contrast to this desired, indurated microstructure, that if the pellets are indurated by direct llame impingement on the pellet bed, this heats at least the surface layer of pellets to so high a temperature that oxidation of the magnetite to hematite does not occur, as a result of which the slag formation is so extensive that the grains of magnetite are embedded in a brittle slag matrix. The microstructure obtained in this way is illustrated in FIG. 8 of the drawings. In this view the magnetite grains .are shown in gray, with a complete slag network, shown in darker gray, surrounding the magnetite grains, the interstices or voids being again shown in black.

What is claimed is:

1. Apparatus for indurating moist, green pellets of comminuted ore particles and the like, comprising: an endless, gas-permeable, travelling grate conveyor encircling spaced means for supporting and propelling the same along a substantially horizontal traverse, means at one end of said conveyor for continuously depositing a bed of said pellets thereon, a gas pressure sustaining housing enclosing said conveyor substantially throughout its length, said housing being compartmentalized into pellet drying, combustion heating, indurating reaction and cooling and reaction zones, an opening between said cooling and reaction zone and said indurating reaction zone compartments, heat generating means disposed in said combustion heating zone compartment, an ambient air intake conduit opening into ysaid cooling and reaction zone compartment, a conduit interconnecting said cooling and reaction zone and said combustion heating zone compartments, a conduit interconnecting said indurating reaction and drying Zone compartments, atmospheric exhaust conduits extending from said combustion heating and drying zone compartments, respectively, and power actuated blower means interposed in each of said conduits for propelling gases therethrough.

2. Apparatus for indurating moist, green pellets of comminuted ore particles and the like, comprising: a gasperrneable, endless travelling grate conveyor encircling spaced means `for supporting and propelling the same along a substantially horizontal traverse, means at one end of said conveyor `for continuously depositing a bed of said pellets thereon, a gas pressure sustaining housing enclosing said conveyor substantially throughout its length, said housing being compartmentalized above and below said conveyor into pellet drying, combustion heating, indurating reaction and cooling and reaction zone compartments, heat generating means dispos-ed in said combustion 'heating Zone compartment above said conveyor, an ambient air intake opening into said cooling and reaction compartment ibeneath said conveyor, a conduit interconnecting said cooling and reaction zone and said combustion heating zone compartments above said conveyor, a conduit interconnecting said indurating reaction and drying zone compartments beneath said conveyor, an atmospheric exhaust conduit extending from said combustion heating Zone compartment beneath said conveyor, an atmospheric exhaust conduit extending from said drying zone compartment above said conveyor, and each of said conduits having interposed therein power actuated blower means for propelling gases therethrough.

3. Apparatus `for indurating moist, lgreen pellets of comminuted ore particles and the like, comprising: a gaspermeable, endless, travelling grate conveyor encircling spaced means for supporting and propelling the same along a su-bstantially horizontal traverse, means at one end of said conveyor 'for continuously depositing a bed of said pellets thereon, a series of funnel-like wind boxes extending along and opening beneath said conveyor substantially throughout its length, hood means disposed above said conveyor and extending substantially throughout its length, said hood means lhaving oppositely disposed sidewall portions extending down below said travelling grate conveyor for providing in conjunction therewith a gas pressure sustaining tunnel-like housing enclosing said conveyor substantially throughout its length, a series of manifolds disposed seriatum beneath said Wind boxes, and a conduit extending from each wind Ibox to the manifold disposed beneath the same, said manifolds serving to apportion said wind boxes along drying, combustion heating, indurating reaction and cooling and reaction zone sections of said conveyor, said hood means being transversely partitioned to provide corresponding drying and comb-ustion heating zone sections thereof and also a corresponding hood section spanning said indurating reactio-n and cooling and .re-action zone sections, an ambient air intake conduit opening into said cooling and reaction Zone manifold, a conduit extending from a .point of said hood means above said cooling and reaction zone mani- -fold to the combustion heating Zone section thereof, a conduit interconnecting the indurating reaction zone manifold and the drying zone manifold, an atmospheric exhaust conduit extending from the combustion heating zone manifold, an atmospheric exhaust conduit extending from the drying zone section of said hood means, and each of said conduits having interposed therein power actuated blower lmeans `for propelling gases therethrough.

d. Apparatus for indurating moist, green pellets of comminuted ore particles and the like, comprising: a gaspermeable, endless, travelling grate conveyor encircling spaced means for supporting and lpropelling the same along a substantial horizontal traverse, means at one end of said conveyor `for continuously depositing a bed of said pellets thereon, la series of `funnel-like wind boxes extending along and opening beneath said lconveyor substantially throughout its length, hood means disposed above said conveyor and extending substantially throughout its length, said hood means having oppositely disposed sidewall portions extending down below said travelling grate conveyor for providing in conjunction therewith a gas pressure sustaining tunnel-like housing enclosing said conveyor substantially throughout its length, a series of manifolds disposed seriatum beneath said wind boxes, and a conduit extending from each wind box to the manifold disposed beneath the same, said manifolds serving to apport'ion said wind boxes along drying, combustion 'heating indurating reaction and cooling and reaction Zone sections of said conveyor, said hood means being transversely partitioned to provide downdraft and updraft drying and combustion heating zone sections thereof and also a corresponding hood section spanning said indurating reaction and cooling and reaction zone sections, an ambient air intake conduit opening into said cooling and reaction lzone manifold, a conduit extending from a point of said hood ymeans above said cooling and reaction zone manifold to the combustion heating zone and downdraft drying zone sections thereof, a conduit interconnecting the indurating reaction zone manifold and the drying zone manifold, -an atmospheric exhaust conduit extending from the combustion heating zone manifold, an atmospheric exhaust conduit extending from the updraftdryin-g zone section of said hood means, and each of said conduits having interposed therein power actuated blower means for propelling Igases therethrough.

References Cited by the Examiner UNITED STATES PATENTS 2,750,272 6/1956 Lellep 263-28 2,750,274 6/1956 Lellep 263-28 3,088,723 5/1963 Haley et al 75--5 X 3,172,754 3/1965 Anthes et al 266-21 X FREDERICK L. MATTESON, JR., Primary Examiner.

JOHN I. CAMBY, Examiner. 

1. APPARATUS FOR INDURATING MOIST, GREEN PELLETS OF COMMINUTED ORE PARTICLES AND THE LIKE, COMPRISING: AN ENDLESS GAS-PERMEABLE, TRAVELLING GRATE CONVEYOR ENCIRCLING SPACED MEANS FOR SUPPORTING AND PROPELLING THE SAME ALONG A SUBSTANTIALLY HORIZONTAL TRAVERSE, MEANS AT ONE END OF SAID CONVEYOR FOR CONTINUOUSLY DEPOSITING A BED OF SAID PELLETS THEREON, A GAS PRESSURE SUSTAINING HOUSING ENCLOSING SAID CONVEYOR SUBSTANTIALLY THROUGHOUT ITS LENGTH, SAID HOUSING BEING COMPARTMENTALIZED INTO PELLET DRYING, COMBUSTION HEATING, INDURATING REACTION AND COOLING AND REACTION ZONES, AND OPENING BETWEEN SAID COOLIN GAND REACTION ZONE AND SAD INDURATING REACTION ZONE COMPARTMENTS, HEAT GENERATING MEANS DISPOSED IN SAID COMBUSTION HEATING ZONE COMPARTMENT, AN AMBIENT AIR INTAKE CONDUIT OPENING INTO SAID COOLING AND REACTION ZONE COMPARTMENT, A CONDUIT INTERCONNECTING SAID COOLING AND REACTION ZONE AND SAID COMBUSTION HEATING ZONE COMPARTMENTS, A CONDUIT INTERCONNECTING SAID INDURATING REACTION AND DRYING ZONE COMPARTMENTS, ATMOSPHERIC EXHAUST CONDUITS EXTENDING FROM SAID COMBUSTION HEATING AND DRYING ZONE COMPARTMENTS, RESPECTIVELY, AND POWER ACTUATED BLOWER MEANS INTERPOSED IN EACH OF SAID CONDUITS FOR PROPELLING GASES THERETHROUGH. 